Locking mechanism for a surgical instrument

ABSTRACT

A surgical instrument has first and second components that can rotate relative to one another and a locking mechanism to selectively restrict or allow rotation between the first and second components. One component has a first array of two or more grooves or teeth spaced apart at a first angular pitch and at least one further groove or further tooth which does not correspond to the first angular pitch. The other component comprises a first tooth or a first groove to engage one of the first array of grooves or teeth to selectively restrict rotation to two or more predetermined rotational positions; the other component also selectively engages the at least one further groove or further tooth on the first component to selectively restrict rotation between the first and second components at a 0° rotational position.

CROSS REFERENCE TO RELATED APPLICATION

This application is a National Stage 35 U.S.C. 371 application ofInternational Patent Application PCT/GB2011/050582 filed Mar. 23, 2011.

BACKGROUND OF THE INVENTION

The present invention relates generally to a surgical instrument havinga locking mechanism arranged to restrict rotational movement between twocomponents. Particular embodiment of the present invention relate tofemoral sizing guides for accurately positioning a cutting block on aresected distal portion of a femur in order to locate cutting planes forpreparing the end of the femur to receive a prosthetic implant. Thepresent invention also relates to methods of using the surgicalinstruments, and in particular femoral sizing guides.

During the lifetime of a patient, it may be necessary to perform a jointreplacement procedure on the patient as a result of, for example,disease or trauma. The joint replacement procedure, or jointarthroplasty, may involve the use of a prosthesis which is implantedinto one of the patient's bones.

During performance of a joint replacement procedure, it is generallyimportant to provide the orthopaedic surgeon with a certain degree offlexibility in the selection of the correct size of prosthesis. Inparticular, the anatomy of the bone into which the prosthesis is to beimplanted may vary somewhat from patient to patient. In order to implanta prosthetic joint, it is commonly necessary to prepare the bone toreceive the prosthesis. For a prosthetic knee joint, both the distalfemur and the proximal tibia may need to be accurately resected to shapethe ends of the bones to receive the implants. For preparing the distalfemur as a first preparatory step, a transverse surface is formed at thedistal end of the femur by performing a first resection. This resectionmay be located using separate instruments, not described in detail here.

Femoral knee prostheses are typically provided in a range of standardsizes. Once selected, the femoral knee prosthesis must be located andoriented to provide appropriate rotational alignment. Correct selectionof the size of prosthesis and correct positioning of the prosthesisrelative to the natural bone is essential to ensure natural movement ofthe assembled joint. In particular, the implant must be positioned toprovide an appropriate gap between the femur and the tibia when the kneeis in extension and in flexion, and to ensure that the surroundingtissues are correctly balanced. It is known to set the rotation of thefemoral prosthesis relative to Whiteside's line, which extends from theintercondylar notch to the patella groove. Alternatively, the rotationof the femoral prosthesis may be set relative to the transepicondylaraxis, which connects the high points of the epicondyles. Furtheranatomical reference marks may also be used.

BRIEF SUMMARY OF THE INVENTION

It is known to use a femoral sizing guide mounted on the resected distalfemur surface to measure the size of the distal femur, and in particularthe size of the condyles and to specify the required locations ofguiding apertures into the femur to secure an appropriate cutting blockto the femur to prepare the femur to receive the prosthesis. A surgicalinstrument set for performing a knee replacement procedure including afemoral sizing guide is marketed by DePuy Orthopaedics, Inc. under thetrade mark Sigma High Performance Instruments. The Sigma HP instrumentset allows surgeons to perform total knee arthroplasty procedures undermost surgical approaches.

The Sigma HP femoral sizing guide is used to determine the correct sizefor the femoral prosthesis and to position guide pins to support acutting block on the distal end of the femur. A body portion of thefemoral sizing guide is seated upon the resected femoral surface.Posterior feet extend from the body underneath the posterior condyles.The femoral sizing guide is correctly located when the posteriorcondylar surfaces rest upon the feet with Whiteside's line extendinggenerally centrally through the sizing guide. When the feet arecorrectly located the body is can be secured to the bone with pins whichextend into the bone through fixed position pin holes. A stylus iscoupled to the body such that it can be raised and lowered and locked inposition. The stylus tip extends over the anterior cortex of the distalfemur. The stylus can also rotate about an axis extending generallyparallel to the resected surface and can slide through the coupling tothe body along the anterior cortex generally parallel to thelongitudinal axis of the femur. The tip of the stylus is positioned uponthe anterior cortex of the femur at the intended exit point of theanterior cut for the femoral prosthesis. The height of the stylus abovethe condylar feet can be read off a scale upon the body and indicatesthe size of the required prosthesis. A scale on the stylus indicates theexit point on the anterior cortex for each size of prosthesis. Thechosen size of the femoral prosthesis determines the size of cuttingblock to be coupled to the distal transverse surface of the femur. Thestylus position sliding through the sizing guide and the height of thestylus above the feet are set to the same value on the respective scales(corresponding to the size of the femoral prosthesis). When the tip ofthe stylus just contacts the anterior cortex as the stylus sweeps acrossthe anterior cortex, the anterior part of the corresponding size offemoral prosthesis will terminate at the anterior surface of the bonewithout leaving a notch or an overhang.

There are two alternative surgical approaches for positioning a femoralprosthesis using the Sigma HP instrument set. These are termed“posterior up” and “anterior down”. Posterior up is based uponaccurately positioning the cutting block relative to the posterior sideof the distal femur (specifically, the posterior condylar surfaces asreferenced by the feet of the sizing guide). Anterior down is based uponaccurately positioning the cutting guide relative to the anterior sideof the distal femur (specifically, the tip of the stylus). For theexisting Sigma HP femoral sizing guide, the positioning of guide pins tosupport the cutting block (for making the anterior, posterior andchamfer cuts) is determined by coupling separate guide blocks to thefemoral sizing guide.

The guide blocks and the position at which they couple to the rest ofthe sizing guide vary according to whether the chosen approach isposterior up or anterior down. Furthermore, the posterior up andanterior down blocks are available in four versions each which relate todifferent degrees of rotation of the femoral prosthesis relative toWhiteside's line or the transepicondylar axis (0°, 3°, 5° and 7° offemoral rotation). For an anterior down approach the guide blocks coupleto the sizing guide close to the stylus and comprise wings which extenddownwards over the transverse distal surface of the femur and includedrill guides for positioning guide pins to support the selected cuttingblock. The guide blocks further comprises a blade which indicatesalignment with Whiteside's line. For a posterior up approach the guideblocks couple to the sizing guide close to the pins securing the body tothe bone, above the feet. The posterior up guide blocks comprise wingswhich extend over the transverse distal surface of the femur and includedrill guides. The guide blocks further comprise a reference surfacewhich indicates alignment with the transepicondylar axis.

Once the cutting block pins have been positioned, the femoral sizingguide can be removed and the appropriate cutting block (according to theselected size of femoral prosthesis) can be positioned over the guidepins to perform the anterior, posterior and chamfer cuts.

In order to position the pins to secure the cutting block for anteriordown and posterior up surgical approaches, and at varying rotationalpositions, the existing Sigma HP femoral sizing guide comprises nineseparate components. The resulting femoral sizing guide is complex andhas multiple parts increasing the expense of the device and increasingthe risk that an incorrect block may be used giving the wrong degree offemoral rotation or the wrong position of guide pins for the selectedcutting block.

U.S. Pat. No. 6,458,135-B1 (assigned to Howmedica Osteonics Corp.)discloses a femoral sizing guide for determining the required size offemoral knee prosthesis and for determining the required location ofalignment holes for securing a cutting block to the distal end of afemur. The femoral sizing guide comprises a sizing block connected to afoot component for selective pivotal movement about a pivot axisextending axially along the femur. The foot component comprises a pairof feet positioned against the posterior condylar surfaces. The centreof rotation is between the feet. A detent mechanism restricts rotationalmovement to predetermined angular positions. A locking mechanism isreleased and the device is manually turned and locked in the requiredposition. The sizing block is positioned upon a resected transversedistal surface of the femur. Rotation of the sizing block relative tothe foot component (and hence rotation of the position of the alignmentholes to be drilled through the sizing block) allows the rotationalposition of the femoral prosthesis about the femur to be varied. Thesize of femoral prosthesis required can be determined either using guidestructures upon the sizing block or via a stylus coupled to the sizingblock.

For the Howmedica device, the position of the alignment holes isdirectly referenced to the posterior condylar surfaces through theselection of appropriate drill guide bushings corresponding to theselected size of femoral prosthesis coupled to the sizing block. For theHowmedica instrument set a single cutting block is used for performingan initial anterior cut, and the position of the anterior cut variesthrough each drill guide bushing adjusting the position of the alignmentholes relative to the foot component. Further adjustment of the positionof the anterior cut is provided through a sliding coupling between thesizing block and the foot component. The Howmedica femoral sizing blocktherefore only allows a surgeon to follow a posterior up surgicalapproach.

U.S. Pat. No. 7,488,324-B1 (assigned to Biomet ManufacturingCorporation) discloses a modular femoral sizing guide which facilitatesthe selection and orientation of a femoral knee prosthesis. A baseportion is coupled to a resected transverse distal surface of the femur.An extension portion has a pair of feet to be positioned under theposterior condylar surfaces. The extension portion is rotatably coupledto the base portion. The centre of rotation is between the feet. Asuperstructure portion has a pair of drilling guides to preparealignment holes extending axially into the femur to couple a cuttingblock to the distal femur. The superstructure further comprises a stylusto determine the required size of the femoral prosthesis.

To position the alignment holes, the Biomet femoral sizing guidecomprises a first actuator to rotate the extension portion relative tothe base portion (thereby rotating the drill guides about the femoralaxis). The sizing guide further comprises a second actuator to adjustthe height of the superstructure relative to the base portion (therebyraising or lowering the drill guides relative to the feet).

For the Biomet device, the position of the alignment holes is referencedto the stylus tip by control of the second actuator to lower the stylusand hence the drill guides until the tip of the stylus contacts theanterior cortex. Consequently, the Biomet femoral sizing block thereforeonly allows a surgeon to follow an anterior down surgical approach.

It is an object of embodiments of the present invention to obviate ormitigate one or more of the problems associated with the prior art,whether identified herein or elsewhere. In particular, it is an objectof embodiments of the present invention to provide a femoral sizingguide for selecting a femoral prosthesis and determining the locationand orientation of the prosthesis upon the distal end of the femur.

According to a first aspect of the present invention there is provided asurgical instrument comprising: a first component having a first face; asecond component coupled to the first component such that the first andsecond components can rotate relative to one another; and a lockingmechanism comprising a third component having a second face coupled tothe second component, movement of the third component relative to thesecond component being arranged to bring the second face into or out ofengagement with the first face so as to selectively restrict or allowrotation between the first and second components; wherein one of saidfaces comprises a first array of two or more grooves or teeth spacedapart at a first angular pitch about the centre of rotation between thefirst and second components and at least one further groove or furthertooth which does not correspond to the first angular pitch; and whereinthe other of said faces comprises a first tooth or a first groovearranged to engage one of the first array of grooves or teeth so as toselectively restrict rotation to two or more predetermined rotationalpositions within a first range of rotational positions between the firstand second components, said other of said faces being arranged to engagethe at least one further groove or further tooth so as to selectivelyrestrict rotation between the first and second components at a furtherpredetermined rotational position.

Advantageously, surgical instruments in accordance with embodiments ofthe present invention allow first and second rotating components to belocked relative to one another at predetermined positions. Particularembodiments relate to femoral sizing guides which assist a surgeon inthe intraoperative selection of a femoral prosthesis and in determiningthe location and orientation of the prosthesis upon the distal end ofthe femur. Advantageously, the orientation of the femoral prosthesisabout the longitudinal axis of the femur may be set using the femoralsizing guide.

The third component may comprise a lever pivotally coupled to the secondcomponent.

Said first array may comprise an array of two or more grooves and saidfirst tooth or first groove may comprise a first tooth.

Said first array and said at least one further groove or further toothmay be positioned on said first face of said first component and saidfirst tooth or first groove may be positioned on said second face ofsaid second component.

In one embodiment the present invention provides a surgical instrumentcomprising: a first component; a second component coupled to the firstcomponent such that the first and second components can rotate relativeto one another; and a locking mechanism comprising a lever coupled tothe second component, movement of the lever relative to the secondcomponent being arranged to bring the lever into or out of engagementwith the first component so as to selectively restrict or allow rotationbetween the first and second components; wherein the first componentcomprises a first array of two or more grooves and the lever comprises afirst tooth arranged to engage one of the first array of grooves so asto selectively restrict rotation between the first and second componentsat two or more predetermined rotational positions within a first rangeof rotational positions between the components; wherein the firstcomponent comprises at least one further groove, the lever beingarranged to engage the at least one further groove so as to selectivelyrestrict rotation between the first and second components at a furtherpredetermined rotational position within a second range of rotationalpositions between the components; and wherein the first array of two ormore grooves comprises grooves spaced apart at a first angular pitchabout the centre of rotation between the first and second components andthe rotational position of the at least one further groove does notcorrespond to the first angular pitch.

The at least one further groove or further tooth may define a 0°rotational position between the components and the first array of two ormore grooves or teeth define at least rotational positions which are 3°,5° and 7° offset from the 0° rotational position.

The first range of rotational positions may extend either side of thefurther rotational position.

The first array of two or more grooves or teeth may be interrupted atthe further rotational position.

The said other face may be arranged to engage the first of said firstand second faces, the said other face may comprise a second array ofteeth or grooves spaced apart at the first angular pitch and arranged toengage the first array of two or more grooves, the width of said otherface being less than the width of the interruption in the first array oftwo or more grooves or teeth at the further rotational position.

One of the array of teeth or grooves may be arranged to engage the atleast one further groove or teeth to selectively restrict rotationbetween the first and second components at the further predeterminedrotational position.

Said one of the second array of teeth or grooves may be longer than theremaining teeth or grooves such that it may engage the at least onefurther groove or tooth while the remaining teeth or grooves do notextend to the at least one further groove.

According to a second aspect of the present invention there is provideda femoral sizing guide for locating an alignment axis in a resectedfemoral surface at a distal end of a femur which is transverse to thelongitudinal axis of the femur, the femoral sizing guide comprising: asurgical instrument as described above; wherein the first componentcomprises a foot component having a first surface arranged to restagainst the resected femoral surface, the foot component furthercomprising first and second feet arranged to extend underneathrespective posterior condyles and rest against posterior condylarsurfaces of the femur, and the second component comprises an extensioncomponent; wherein the femoral sizing guide further comprises: asuperstructure coupled to the extension component and arranged to slideparallel to the resected femoral surface towards and away from the feet;and a stylus coupled to the superstructure and arranged such that whenthe first surface of the foot component rests against the resectedfemoral surface a tip of the stylus extends over the femur such thatsliding the superstructure towards the feet causes the stylus tip tocontact the anterior cortex of the femur; wherein the superstructurefurther comprises a first guide hole defining a first alignment axisextending into the resected femoral surface at a predetermined distancefrom the level of the stylus tip in the plane of the resected femoralsurface; and wherein the extension component defines a second guide holedefining a second alignment axis extending into the resected femoralsurface such that the distance between the first and second guide holesvaries as the superstructure slides relative to the body.

The superstructure may comprise a first pair of guide holes defining afirst pair of alignment axes and the extension component defines asecond pair of guide holes defining a second pair of alignment axes, andwherein the guide holes comprise drill guides for drilling holes intothe resected femoral surface or are arranged to guide pins extendinginto the resected femoral surface along the alignment axes.

The extension component may be arranged to rotate relative to the footcomponent about an axis which extends into the resected femoral surface,the axis of rotation lying on a line which extends between the feet inthe plane of the first surface of the foot component.

The second pair of guide holes may comprise guide hole sleeves extendingfrom the extension component and through one or more arcuate slotswithin the foot component, the or each arcuate slot extending along acurve which has a first radius of curvature about the centre of rotationbetween the foot component and the extension piece such that the guidehole sleeves can slide along the or each arcuate slot.

The femoral sizing guide may further comprise a retaining bar extendingbetween the guide hole sleeves such that the foot component is trappedbetween the retaining bar and the extension piece to couple theextension piece to the foot component.

The foot component may further comprise a pair of fixing holes arrangedto receive a fixation pin to secure the foot component to a resectedsurface of a bone, the fixing holes comprising fixing hole sleevesextending from the foot component and through one or more arcuate slotswithin the extension component, the or each arcuate slot extending alonga curve which has a second radius of curvature about the centre ofrotation between the foot component and the extension piece such thatthe fixing hole sleeves can slide along the or each arcuate slot, thesecond radius of curvature being different to the first radius ofcurvature.

The femoral sizing guide may further comprise a spigot extending fromeither the foot component or the extension piece and received within anarcuate slot formed within the other of the foot component and theextension piece, the arcuate slot extending along a curve which has athird radius of curvature about the centre of rotation between the footcomponent and the extension piece such that the spigot can slide alongthe arcuate slot, the third radius of curvature being different to thefirst and second radii of curvature.

The first pair of alignment axes may lie in a first plane and the secondpair of alignment axes may lie in a second plane such that the distancebetween the planes varies as the superstructure slides relative to thebody.

The first and second planes may be parallel and the body may comprise afirst surface arranged to bear against the resected femoral surface,wherein the alignment axes are arranged to extend into the resectedfemoral surface at a predetermined angle.

The guide holes may comprise drill guides for drilling holes into theresected femoral surface or may be arranged to guide pins extending intothe resected femoral surface along the alignment axes.

The foot component may further comprise at least one fixing holearranged to receive a fixation pin to secure the foot component to aresected surface of a bone.

The superstructure may comprise a head part and first and second armsextending from the head part towards the body such that they are insliding contact with the body, the arms defining the or each first guidehole.

The femoral sizing guide may further comprise a support rod extendingfrom the body and received in a bore in the head part of thesuperstructure such that as the superstructure slides towards and awayfrom the feet the support rod passes through the bore.

The femoral sizing guide may further comprise a locking knob coupled tothe head part of the superstructure and arranged to lock thesuperstructure to the support rod to restrict further sliding of thesuperstructure relative to the body.

The stylus may comprise an elongate slot arranged to couple to the headpart of the superstructure such that the stylus extends from thesuperstructure transverse to the support rod, the stylus slot beingarranged to allow the stylus to slide relative to the support rod and torotate about the support rod.

The stylus comprises a first portion which extends transverse to thesupport rod and a second portion which is inclined relative to the firstportion such that stylus tip points towards an anterior cortex of thefemur.

According to a third aspect of the present invention there is provided amethod of selectively restricting or allowing rotation between first andsecond components of a surgical instrument, the method comprising:coupling a first component having a first face to a second componentsuch that the first and second components can rotate relative to oneanother; coupling a locking mechanism comprising a third componenthaving a second face to the second component such that the thirdcomponent can move relative to the second component to bring the secondface into or out of engagement with the first face so as to selectivelyrestrict or allow rotation between the first and second components;wherein one of said faces comprises a first array of two or more groovesor teeth spaced apart at a first angular pitch about the centre ofrotation between the first and second components and at least onefurther groove or further tooth which does not correspond to the firstangular pitch; and wherein the other of said faces comprises a firsttooth or first groove arranged to engage one of the first array ofgrooves or teeth so as to selectively restrict rotation to two or morepredetermined rotational positions between the first and secondcomponents, said other of said faces being arranged to engage the atleast one further groove or further tooth so as to selectively restrictrotation between the first and second components at a furtherpredetermined rotational position.

In one embodiment the present invention provides a method of selectivelyrestricting or allowing rotation between first and second components ofa surgical instrument, the method comprising: coupling a first componentto a second component such that the first and second components canrotate relative to one another; coupling a locking mechanism comprisinga lever to the second component such that the lever can move relative tothe second component to bring the lever into or out of engagement withthe first component so as to selectively restrict or allow rotationbetween the first and second components; wherein the first componentcomprises a first array of two or more grooves and the lever comprises afirst tooth arranged to engage one of the first array of grooves so asto selectively restrict rotation between the first and second componentsat two or more predetermined rotational positions within a first rangeof rotational positions between the components; and wherein the firstcomponent comprises at least one further groove, the lever beingarranged to engage the at least one further groove so as to selectivelyrestrict rotation between the first and second components at a furtherpredetermined rotational position within a second range of rotationalpositions between the components.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompany drawings, in which:

FIGS. 1 and 2 illustrate first and second views of a femoral sizingguide in accordance with an embodiment of the present invention fromdifferent perspectives;

FIGS. 3 and 4 illustrate exploded views of the femoral sizing guide ofFIGS. 1 and 2 from different perspectives;

FIGS. 5A and 5B illustrate enlarged views of parts of the lockingmechanism of the femoral sizing guide of FIGS. 1 and 2; and

FIG. 6 illustrates the femoral sizing guide of FIG. 1 coupled to aresected surface at the end of a femur.

DETAILED DESCRIPTION OF THE INVENTION

Before a femoral sizing guide in accordance with an embodiment of thepresent invention can be used, the distal end of the femur must undergoinitial resection to remove a distal portion of the femur to establish asurface which is transverse to the longitudinal axis of the femur. Thefemoral sizing guide is arranged to either allow alignment holes to bedrilled into the resected surface or to place guide pins extending fromthe resected surface. Specifically, femoral sizing guides in accordancewith embodiments of the present invention include drill guide holeswhich allow either holes to be drilled into the resected surface of thebone or guide pins to be directly inserted into the bone passing throughthe drill guides. The alignment holes or guide pins are used to positiona cutting block upon the surface of the bone to perform the shapingresections of the end of the femur. Consequently, the position of thealignment holes/guide pins determined by drill guides on the femoralsizing guide relative the condyles/anterior cortex and their rotationalposition about the axis of the femur determine the final implantedposition of the femoral prosthesis.

As noted above in connection with the known Sigma HP femoral sizingguide, there are two principal options for positioning a cutting blockto prepare a distal end of a femur to receive a femoral knee prosthesis:anterior down and posterior up. The choice is dependent partially uponsurgeon preference. An anterior down approach prevents notching of theanterior cortex of the distal femur or the femoral prosthesisoverhanging at the expense of less control over the position of theprosthetic posterior condyles. A posterior up approach allows accuratecontrol of the position of the posterior condyles and therefore bettercontrol of the joint tension when the knee is in flexion. The femoralprostheses may be provided in a range of sizes with regular sizeincrements. For instance, the femoral prostheses may be provided inincrements of 3 mm, though it will be appreciated that other incrementsare possible. Femoral sizing guides in accordance with embodiments ofthe present invention are required to locate guide holes or pinsextending into the resected surface at a constant position relative toeither the anterior cortex or the posterior condyles irrespective of theselected size of the femoral prosthesis. The stylus provides a guide tothe appropriate size implant and, therefore, cutting block to use. Asthe position of the guide pins is selected using the femoral sizingguide, the cutting blocks can be designed to have a fixed distancebetween the pin holes and the position of the bone cut, rather thanrequiring that the cutting blocks allow the position of the bone cut tobe adjusted relative to the pin positions. The cutting block may providefurther adjustment means allowing the surgeon to control the position ofthe cutting block relative to the anterior cortex or posterior condyles.For an exemplary range of femoral prosthesis having 3 mm sizeincrements, the cutting blocks may be arranged to allow the position ofeach cutting block relative to the guide pin or guide holes (andtherefore relative to the anterior cortex or posterior condyles) towithin + or −1.5 mm. This allows for flexibility to accommodate femurswhich fall between standard sizes of the femoral prosthesis. The cuttingblock adjustment means may comprise a series of three holes to receiveeach guide pin. A first hole is at the normal position and the otherholes are 1.5 mm above and below the first hole.

FIGS. 1 and 2 illustrate perspective views of a femoral sizing guide inaccordance with an embodiment of the present invention. FIGS. 3 and 4illustrate exploded views. The femoral sizing guide comprises a footcomponent 2 and an extension piece 4, collectively forming a body 6. Thefoot component 2 and the extension piece 4 are rotatably coupledtogether as will be described in greater detail below. The footcomponent 2 comprises a first side 8 (visible in FIGS. 2 and 4) which inuse is placed against the resected distal surface of the femur.Similarly the extension piece 4 also has a first surface 10 (visible inFIGS. 2 and 4) to bear against the femur. The foot component 2 comprisesfirst and second feet 12 which in use extend underneath the posteriorcondyles. In particular, foot locator surfaces 14 are arranged to bearagainst respective posterior condylar surfaces to locate the femoralsizing guide on the resected femoral surface. The foot component 2further comprises first and second pin holes 16. When the femoral sizingguide is correctly positioned on the resected femoral surface, such thatthe foot locator surfaces bear against respective posterior condylarsurfaces, fixing pins are driven into the resected bone surface throughpin holes 16 to secure the femoral sizing guide in place to preventfurther movement of the foot component 2 relative to the femur.

The extension piece 4 is rotatably coupled to the foot component 2 suchthat when the foot component 2 is pinned to the femur the extensionpiece can rotate by sliding over the bone surface. The centre ofrotation is generally positioned between the feet 12 such that when thefemoral sizing guide is pinned to the bone the centre of rotation isgenerally aligned with Whiteside's line. However, there is no pivotalconnection at the centre of rotation. Advantageously, this allows thesurgeon a clear view of Whiteside's line, which assists with determiningthe required rotational alignment of the femoral prosthesis. Theextension piece 4 comprises first and second arcuate grooves 18 whichare defined by a curve radiating from the centre of rotation of the footcomponent 2 and the extension piece 4. Each groove 18 is arranged toreceive an extended sleeve portion 20 of a foot component pin hole 16such that as the extension piece 4 rotates relative to the footcomponent 2 its movement is constrained by the pin holes sleeves 20sliding through grooves 18. The range of rotational movement of theextension piece 4 relative to the foot component 2 is limited by the pinhole sleeves 20 bearing against closed outer ends of the arcuate grooves18.

The extension piece 4 further comprises a first pair of guide holes 22for determining the axes of alignment holes or guide pins extending intothe resected femur for a posterior up approach, as will be described ingreater detail below. The guide holes 22 comprise sleeve portions 24which extend through arcuate grooves 26 formed in the foot component 2.The foot component slots 26 are open ended as they extend to the edge ofthe foot component 2 and are defined by a curve having a larger radiusof curvature than the radius of curvature for the extension piece slots18. Slots 18 and 26 extend about the same centre of curvature. As theextension piece 4 rotates relative to the foot component 2 the guidehole sleeves 24 slide along foot component grooves 26. The range ofrotational movement of the extension piece 4 relative to the footcomponent 2 is further limited by the guide hole sleeves 24 bearingagainst the closed ends of foot component grooves 26.

To secure the extension piece 4 to the foot component 2 a retaining bar28 extends between the guide hole sleeves 24 such that the footcomponent 2 is sandwiched between the extension piece 4 and theretaining bar 28. To provide further stability to the assembled femoralsizing guide, a spigot 29 (not visible in FIGS. 1 and 2) extends fromthe foot component 2 into an arcuate slot 30 formed in the extensionpiece 4 such that the spigot 29 slides within slot 30 as the extensionpiece rotates relative to the foot component 2. The arcuate slot 30 isdefined by a curve radiating from the centre of rotation between thefoot component 2 and the extension piece 4.

A locking mechanism is provided to lock the extension piece 4 to thefoot component 2 at predetermined rotational positions. The lockingmechanism comprises a lever 32 which is pivotally coupled to theextension piece 4. The lever 32 is operated by trigger 34, specificallyby squeezing trigger 34 against bar 36 which extends outwardly from theextension piece 4. The lever 32 is biased towards the foot component 2by a spring (not illustrated) which extends between lugs 37 formed onthe trigger 34 and the bar 36 such that when the trigger 34 is releasedthe lever bears against the foot component 2 to prevent further rotationof the extension piece. The lever 32 comprises a groove 38 which couplesto a bar 39 formed within the extension piece 4 to allow the lever torotate relative to the extension piece 4. It will be appreciated thatalternative mechanisms for coupling the lever 32 to the extension piece4 are possible, the only requirement being that relative movementbetween the lever 32 and the foot component 2 is provided to allowinglocking and unlocking. For instance, a pin may be provided press fittedinto a hole formed in lever 32 generally at the position of groove 38.The pin may be received and rotate within a hole in extension piece 4.The locking mechanism is described in greater detail below.

The femoral sizing guide further comprises a superstructure 40 which iscoupled to the extension piece 4 and arranged to slide relative to theextension piece 4 generally towards and away from the feet 12. Thesuperstructure 40 comprises a head part 42 and arms 44 which extenddownwardly and are arranged to slide within channels 46 formed in thesides of the extension piece 4. A support rod 48 extends upwards fromthe central part of the extension piece 4 and passes through thesuperstructure head 42. The support rod 48, together with arms 44sliding in channels 46 serve to control sliding movement of thesuperstructure 40 and to prevent twisting of the superstructure 40relative to the extension piece 4 as it slides. The superstructure 40can be locked in position relative to the extension piece 4 bytightening locking knob 50, which compresses a collet 51 (visible inFIGS. 3 and 4) over the support rod 48.

The superstructure 40 further comprises a second pair of guide holes 52positioned towards the free ends of the arms 44. The second pair ofguide holes 52 is for determining the axes of alignment holes extendinginto the resected femur for an anterior down approach, as will bedescribed in greater detail below. Superstructure guide holes 52 areused as an alternative to the extension piece guide holes 22, as will bedescribed below as part of the surgical technique for using the femoralsizing guide. The superstructure guide holes 52 comprise sleeves suchthat in use the tips of the sleeves are contact with the resected bonesurface.

The femoral sizing guide further comprises a stylus 53 coupled to thehead part 42 of the superstructure 40. The stylus 53 is arranged toextend over the distal end of the femur when the femoral sizing guide iscoupled to the resected distal surface of the femur. The stylus 53 iscoupled to the superstructure 40 such that it extends from head part 42substantially at 90° to the axis of support rod 48. The stylus 53comprises an elongate slot 54 arranged to engage a neck 56 of the headpart 42 so that the stylus can slide relative to the head part 42 andcan rotate about the body part 42. The stylus slot 54 further comprisesan enlarged portion 57 arranged to pass over the upper side of head part42 and the locking knob 50 to detach the stylus 53 from the femoralsizing guide. The stylus 53 further comprises a handle 58 at a first endto rotate the stylus 53 relative to the superstructure 40 and to slidethe slot 54 over the neck 56 of the superstructure 40.

At the second end, the stylus 53 is bent downwards towards a stylus tip60. The stylus tip 60 is arranged to contact the anterior cortex of thefemur by sliding and rotating the stylus 53 relative to thesuperstructure 40 and by sliding the superstructure 40 relative to theextension piece 4. The stylus 53 has a scale 62 marked along both sidesof slot 54. Each mark on the scale 62 corresponds to a selected size offemoral prosthesis. The position of the stylus is indicated by the markon the stylus scale 62 adjacent to head part 42 on the stylus handleside of the body part 42. Similarly, the extension piece 4 furthercomprises a scale 64 marked along both sides of the extension piece 4.Each mark on the extension piece scale 64 corresponds to a selected sizeof femoral prosthesis. The position of the superstructure 40 relative tothe extension piece 4 is indicated by the mark on the superstructurescale 64 adjacent to the centre of the superstructure guide holes 52.Stylus scale 62 and superstructure scale 64 both indicate the same rangeof sizes of femoral prosthesis. When the superstructure 40 and thestylus 53 are both positioned at corresponding locations on scales 64,62 then the stylus tip 60 indicates the exit point on the anteriorcortex for the selected size of femoral prosthesis. Both thesuperstructure 40 and the stylus 53 are adjusted according to theirrespective scales until the stylus tip 60 is just in contact with theanterior cortex of the bone as the stylus tip 60 is swept across theanterior cortex. The scales 62, 64 then indicate the required size offemoral prosthesis, as will be described in greater detail below inconnection with the remainder of the surgical technique for using thefemoral sizing guide.

Referring now to FIGS. 5A and 5B these illustrate enlarged portions ofthe sizing guide locking mechanism in order that the operation of thelocking mechanism may be more clearly understood. FIG. 5A illustrateslever 32 separated from the extension piece 4. Lever 32 is illustratedfrom the same perspective as the exploded view of FIG. 3. FIG. 5Billustrates part of the foot component 2 separated from the extensionpiece 4. The foot component 2 is illustrated from the same perspectiveas the exploded view of FIG. 4. The lever 32 of FIG. 5A and the footcomponent of FIG. 5B are viewed from different perspectives so as toallow the portions of each component that face one another to be viewed.

As noted above, the lever 32 is coupled to the extension piece 4 suchthat it can rotate into and out of contact with the foot component 2 tolock the rotational position of the extension piece 4 relative to thefoot component 2. The lever 32 is biased towards the foot component 2,but the bias can be overcome by the surgeon squeezing trigger 34 towardsbar 36. The lever 32 has a lever face 70 which bears against a ratchetplate 72 on the foot component 2. The lever face 70 has an array ofteeth 74 which bear against a curved ratchet 76 on the ratchet face 72.When the trigger 34 is squeezed against bar 36 teeth 74 are disengagedfrom the curved ratchet 76. The extension piece 4 can be freely rotatedrelative to the foot component 2 within a predetermined rotational rangelimited by movement of sleeves 20 and 24 within respective slots 18 and26.

When the trigger 34 is released, the teeth 74 engage the curved ratchet76 causing the orientation of the extension piece 4 relative to the footcomponent 2 to be limited to a series of predetermined rotationalpositions. For instance, the locking mechanism may be arranged to allowthe extension piece 4 to be locked relative to the foot component 2 at0° (which corresponds to a symmetrically upright position). As theextension piece 4 rotates relative to the foot component 2 the verticalrod 48 also rotates. Rod 48 serves as a visual reference to the correctrotational position of the extension piece. The correct rotationalposition of the extension piece 4 relative to the foot component 2 isindicated when rod 48 is aligned with Whiteside's line, which generallycorresponds to an anterior to posterior axis for the distal femur.Additionally, horizontal lines formed in the visible end of guidesleeves 52 serve as a visual reference for aligning the rotation of theextension piece relative to the transepicondylar axis. Rotation of theextension piece 4 relative to the foot component 2 may be locked at+/−3°, 5° and 7° relative to the 0° position. While the lockingmechanism is released the extension piece 4 may be freely rotated untilthe rod 48 is brought into alignment with Whiteside's line. The lockingmechanism may then be engaged. The interaction of the teeth 74 with thecurved ratchet 76 serves to cause the position of the extension piece 4relative to the foot component 2 to settle at the closest position.Advantageously, although the locking mechanism is small, it still allowsaccurate selection of 0° and +/−3°, 5° or 7° of offsets. This selectionis achieved by having two sets of grooves within the curved ratchet 76.An upper ratchet 78 has a 2° pitch and is provided in two separateportions with a space in between. Each portion of the upper ratchet 78is aligned with +/−3°, 5° or 7° grooves relative to a centreline 80. Thegrooves in the upper ratchet 78 are arranged to engage correspondingteeth 74 on the lever face 70. Consequently, the upper ratchet 76 allowsthe orientation of the extension piece 4 relative to the foot component2 to be fixed at predetermined positions (+/−3°, 5° or 7°) within firstportions of the predetermined rotational range indicated by arrows 90.

The curved ratchet 76 further comprises a second, lower ratchet 82 whichcomprises a single groove aligned with the 0° position. It will beappreciated that the second ratchet 82 may overlap with the firstratchet 78, as illustrated in FIG. 5B so long as there is a sufficientdifference in alignment so allow the first ratchet to be engaged withoutinterference from the second ratchet, and vice versa, as will now bedescribed. The lever teeth 74 have an elongated central tooth 84 whichis arranged to engage lower ratchet groove 82 when the lever iscentrally positioned across curved ratchet 76. There is only a singlegroove 84 within a second, central portion of the predeterminedrotational range of the extension piece 4 relative to the foot component2 indicated by arrow 92. Consequently, the extension piece 4 can only belocked relative to the foot component at a single position within thesecond portion 92 of the predetermined rotational range. It will beappreciated that in alternative embodiments of the invention there maybe more than two separation portions of the predetermined rotationalrange, each supporting ratchets which are arranged to be separatelyengaged by the lever 32 to allow the extension piece to be lockedrelative to the foot component. Furthermore, within each portion of therotational range the corresponding ratchet may be arranged to allow theorientation of the extension piece relative to the foot component to belocked at one or more predetermined orientations, according to thenumber of grooves. Groove 82 has broad shoulders preventing the centrallever tooth 84 from settling at other positions within the secondportion 92 of the rotational range. The upper ratchet 78 has a gap inthe second portion 92 of the rotational range. This prevents theremainder of the lever teeth from clashing with the upper ratchet. Asthe central lever tooth 84 comprises an extension of tooth at the samepitch as the remainder of the lever teeth 74 when the lever engages theupper ratchet in the first portions of the rotational range the centraltooth 84 meshes with the upper ratchet 76.

It will be appreciated that because the upper ratchet 78 has a 2° pitchat positions +/−3°, 5° or 7°, it would not be possible for this ratchetto accommodate a 0° position. The provision of a separate ratchetcomprising one or more grooves centred on 0° which is engaged by one ormore extended teeth allows the 0° position also to be selected. It willbe appreciated that alternatively a single ratchet could be providedwith teeth having a 1° pitch. However, a ratchet with 1° teeth wouldeither have very small teeth not able to securely lock the extensionpiece 4 to the foot component 2 or would have to extend over a largearea. The arrangement of two separate ratchets which are separatelyengaged overcomes these problems. It will be appreciated that there areother mechanisms by which this effect could be achieved. For instance,the central groove 82 may be omitted and the whole of the lever face 70including all of the teeth 74 may be arranged to be received in the gapin the centre of the upper ratchet 76 to lock the mechanism at the 0°position. That is, the whole of the gap in the upper ratchet 76 maycomprise at least one further groove. Other arrangements will beapparent to the appropriately skilled person in which a lever may engagea ratchet having first regular pitch at a position not corresponding tothe ratchet pitch.

A method of using a femoral sizing guide in accordance with anembodiment of the present invention will now be described in connectionwith FIG. 6 which illustrates the femoral sizing guide coupled to aresected distal portion of the bone. A foot component 2 is positionedagainst the end of the bone such that surface 8 is in contact with thebone and the feet 12 extend under the posterior condyles. The footcomponent is then moved across the resected surface until the footlocator surfaces 14 bear against the posterior condylar surfaces (thatis, the femoral sizing guide is centrally located on the end of thebone. The femoral sizing guide can then be secured to the bone bypassing fixing pins through holes 16 in the foot component into thebone.

The rotational alignment of the extension piece (and hence therotational alignment of the guide pins for securing the cutting blockand the resulting femoral prosthesis) can then be set. The requiredrotational alignment may be determined during pre-operative planning orintra-operatively using anatomical landmarks, for instance by rotatingthe extension piece 4 relative to the foot component 2 until the rod 48is aligned with Whiteside's Line. The rotation is set by squeezing thetrigger 34 and bar 36 together to release the locking mechanism and thenmanually rotating the extension piece 4 and superstructure 40. When therequired rotation is set and checked on scale 94 on the extension piecerelative to a mark 96 on the foot component, the locking mechanism isengaged by releasing trigger 34.

As discussed above, the sizing guide can be set to varying sizes offemoral prosthesis by adjusting the height of the superstructure 40relative to scale 64 on the extension piece 4. The superstructure 40 israised and lowered by releasing and tightening locking knob 50 to causecollet 51 to grip or release the rod 48. Additionally, the position ofthe stylus 53 is set by sliding stylus slot 54 over extension piece neck56 and reading the stylus position on scale 62. For each femoralprosthesis selected for the stylus on scale 62, the superstructureshould be set to the same size on scale 64. The tip 60 of the stylus ispositioned on the anterior cortex of the femur at the position thesurgeon has chosen for the anterior cut to exit the bone. The selectedsize of femoral prosthesis on scale 62 is then transferred to scale 64by sliding the superstructure 40 relative to the extension piece 4.Ideally for the chosen prosthesis size the stylus tip should just touchthe anterior cortex; however in practice the stylus tip may touch theanterior cortex at a position where the indication of prosthesis size bythe position of the superstructure 40 relative to the extension piece 4is between two standard sizes of prosthesis. Whether the next largest ornext smallest prosthesis is chosen is at the choice of the surgeon.

Guide pins to support the cutting block can then be inserted througheither the lower alignment holes 22 (for a posterior up approach) or theupper alignment holes 52 (for an anterior down approach). The choicebetween anterior down and posterior up approaches is the same as for theSigma HP instrument set described above. Either guide pins are insertedinto the bone through the alignment holes or alignment holes may bedrilled into the bone. The sizing guide is then removed and the guidepins or holes used to mount a cutting guide to perform the shapingresections for the selected size of femoral prosthesis.

Embodiments of the present invention described above primarily relate toa lever coupled to one component of a surgical instrument and arrange tomove to engage another component pivotally coupled to the firstcomponent. However, it will be appreciated that further lockingmechanisms fall within the scope of the invention. For instance, inplace of a lever there may be provide a sliding button or any other suchsuitable mechanism. Additionally, the skilled person will appreciatethat where embodiments of the present invention are described as havingparticular arrangements of teeth and grooves, the invention is broaderthan this. In particular, the respective formations on the lever and afirst component of a surgical instrument may be switched over. Also, ineach instance, teeth and grooves may be reversed, for instance such thata single groove engages one of an array of teeth. More generally, thepresent invention relates to any mechanism for selectively restrictingrotational movement between first and second components to one of apredetermined number of rotational positions, said predetermined numberof rotational positions comprising two or more positions at a firstangular pitch and at least one further position not falling into thefirst angular pitch.

It will be readily apparent to the appropriately skilled person thatfurther modifications may be made to the present invention and furtherapplications may be found for the present invention from the teachingherein, without departing from the scope of the appended claims.

The invention claimed is:
 1. A surgical instrument, comprising: a firstcomponent having a first face; a second component coupled to the firstcomponent such that the first and second components can rotate relativeto one another; and a locking mechanism having a second face coupled tothe second component, movement of the locking mechanism relative to thesecond component being arranged to bring the second face into or out ofengagement with the first face so as to selectively restrict or allowrotation between the first and second components; wherein one of thefirst and second faces comprises a first array of two or more grooves orteeth spaced apart at a first angular pitch about a center of rotationbetween the first and second components and has at least one furthergroove or further tooth that has a different length than the othergrooves or teeth in the first array; wherein the other of the first andsecond faces comprises a first tooth or has a first groove arranged toengage one of the first array of grooves or teeth so as to selectivelyrestrict rotation to two or more predetermined rotational positionswithin a first range of rotational positions between the first andsecond components, the other of the first and second faces beingarranged to engage the at least one further groove or further tooth soas to selectively restrict rotation between the first and secondcomponents at a 0° rotational position between the first and secondcomponents; wherein the first array of two or more grooves or teethdefine rotational positions that are offset from the 0° rotationalposition.
 2. The surgical instrument of claim 1, wherein the lockingmechanism comprises a lever pivotally coupled to the second component.3. The surgical instrument of claim 1, wherein the first array comprisesan array of two or more grooves and said first tooth or first groovecomprises a first tooth.
 4. The surgical instrument of claim 1, whereinthe first array and the at least one further groove or further tooth arepositioned on said first face of the first component and the first toothor first groove is positioned on the second face of the secondcomponent.
 5. The surgical instrument of claim 1, wherein the firstarray of two or more grooves or teeth define at least rotationalpositions that are 3°, 5° and 7° offset from the 0° rotational position.6. The surgical instrument of claim 1, wherein the first range ofrotational positions extends either side of the 0° rotational position.7. The surgical instrument of claim 6, wherein the first array of two ormore grooves or teeth is interrupted at the 0° rotational position, theinterruption having a width.
 8. The surgical instrument of claim 7,wherein the first tooth or groove comprises a second array of teeth orgrooves spaced apart at the first angular pitch and arranged to engagethe first array of two or more grooves or teeth, the second array ofteeth or grooves having a width, the width of the second array of teethor grooves being less than the width of the interruption.
 9. Thesurgical instrument of claim 8, wherein one of the second array of teethor grooves is arranged to engage the at least one further groove orfurther tooth to selectively restrict rotation between the first andsecond components at the further rotational position.
 10. A femoralsizing guide for locating an alignment axis in a resected femoralsurface at a distal end of a femur, wherein the femur has a longitudinalaxis and an anterior cortex and wherein the resected femoral surfacelies in a plane that is transverse to the longitudinal axis of thefemur, the femoral sizing guide comprising: the surgical instrument ofclaim 1, wherein the first component comprises a foot component having afirst surface arranged to rest against the resected femoral surface, thefoot component further comprising first and second feet arranged toextend underneath respective posterior condyles and rest againstposterior condylar surfaces of the femur, and the second componentcomprises an extension component; a superstructure coupled to theextension component and arranged to slide parallel to the resectedfemoral surface towards and away from the feet; and a stylus coupled tothe superstructure and arranged such that when the first surface of thefoot component rests against the resected femoral surface a tip of thestylus extends over the femur such that sliding the superstructuretowards the feet causes the stylus tip to contact the anterior cortex ofthe femur; wherein the superstructure further comprises a first guidehole defining a first alignment axis extending into the resected femoralsurface at a predetermined distance from the level of the stylus tip inthe plane of the resected femoral surface; and wherein the extensioncomponent defines a second guide hole defining a second alignment axisextending into the resected femoral surface such that the distancebetween the first and second guide holes varies as the superstructureslides relative to the body.
 11. The femoral sizing guide of claim 10,wherein the superstructure comprises a first pair of guide holesdefining a first pair of alignment axes and the extension componentdefines a second pair of guide holes defining a second pair of alignmentaxes.
 12. The femoral sizing guide of claim 11, wherein the firstsurface of the foot component lies in a plane and wherein the extensioncomponent is arranged to rotate relative to the foot component about anaxis that extends into the resected femoral surface, the axis ofrotation lying on a line that extends between the feet in the plane ofthe first surface of the foot component.
 13. The femoral sizing guide ofclaim 12, wherein the second pair of guide holes comprise guide holesleeves extending from the extension component and through one or morearcuate slots within the foot component, the or each arcuate slotextending along a curve that has a first radius of curvature about theaxis of rotation between the foot component and the extension componentsuch that the guide hole sleeves can slide along the or each arcuateslot.
 14. The femoral sizing guide of claim 13, further comprising aretaining bar extending between the guide hole sleeves such that thefoot component is trapped between the retaining bar and the extensioncomponent to couple the extension piece to the foot component.
 15. Thefemoral sizing guide of claim 14, wherein the foot component furthercomprises a pair of fixing holes arranged to receive a fixation pin tosecure the foot component to a resected surface of a bone, the fixingholes comprising fixing hole sleeves extending from the foot componentand through one or more arcuate slots within the extension component,the or each arcuate slot extending along a curve that has a secondradius of curvature about the axis of rotation between the footcomponent and the extension component such that the fixing hole sleevescan slide along the or each arcuate slot, the second radius of curvaturebeing different to the first radius of curvature.
 16. The femoral sizingguide of claim 15, further comprising a spigot extending from either thefoot component or the extension component and received within an arcuateslot formed within the other of the foot component and the extensioncomponent, the arcuate slot extending along a curve that has a thirdradius of curvature about the axis of rotation between the footcomponent and the extension component such that the spigot can slidealong the arcuate slot, the third radius of curvature being different tothe first and second radii of curvature.
 17. A surgical instrument,comprising: a first component having a first face; a second componentcoupled to the first component such that the first and second componentscan rotate relative to one another; and a locking mechanism having asecond face coupled to the second component, movement of the lockingmechanism relative to the second component being arranged to bring thesecond face into or out of engagement with the first face so as toselectively restrict or allow rotation between the first and secondcomponents; wherein one of the first and second faces comprises a firstarray of two or more grooves or teeth spaced apart at a first angularpitch about a center of rotation between the first and second componentsand has at least one further groove or further tooth that does notcorrespond to the first angular pitch; wherein the other of the firstand second faces comprises a first tooth or has a first groove arrangedto engage one of the first array of grooves or teeth so as toselectively restrict rotation to two or more predetermined rotationalpositions within a first range of rotational positions between the firstand second components, the other of the first and second faces beingarranged to engage the at least one further groove or further tooth soas to selectively restrict rotation between the first and secondcomponents at a further predetermined rotational position; wherein thefirst range of rotational positions extends either side of the furtherrotational position; wherein the first array of two or more grooves orteeth is interrupted at the further rotational position, theinterruption having a width; wherein the first tooth or groove comprisesa second array of teeth or grooves spaced apart at the first angularpitch and arranged to engage the first array of two or more grooves orteeth, the second array of teeth or grooves having a width, the width ofthe second array of teeth or grooves being less than the width of theinterruption; wherein one of the second array of teeth or grooves isarranged to engage the at least one further groove or further tooth toselectively restrict rotation between the first and second components atthe further rotational position; and wherein the second array of teethor grooves comprises a plurality of teeth or grooves, one of the secondarray of teeth or grooves is longer than another of the teeth or groovessuch that the longer tooth or groove may engage the at least one furthergroove or further tooth while the other tooth or groove of the secondarray does not extend to the at least one further groove or tooth.
 18. Asurgical instrument, comprising: a first component having a first face;a second component coupled to the first component such that the firstand second components can rotate relative to one another; and a lockingmechanism having a second face coupled to the second component, movementof the locking mechanism relative to the second component being arrangedto bring the second face into or out of engagement with the first faceso as to selectively restrict or allow rotation between the first andsecond components; wherein one of the first and second faces comprises afirst array of two or more grooves or teeth and has at least one furthergroove or further tooth that has a different length than another of theteeth or grooves; wherein the other of the first and second facescomprises a second array of two or more grooves or teeth arranged toengage at least one of the first array of grooves or teeth so as toselectively restrict rotation between the first and second components totwo or more predetermined rotational positions; and wherein one grooveor tooth of the second array is longer than another of the grooves orteeth in the second array so as to selectively engage the one furthergroove or further tooth that has a different length than another of theteeth or grooves.
 19. A femoral sizing guide for locating an alignmentaxis in a resected femoral surface at a distal end of a femur that istransverse to the longitudinal axis of the femur, the femoral sizingguide comprising: the surgical instrument of claim 18, wherein the firstcomponent comprises a foot component having a first surface arranged torest against the resected femoral surface, the foot component furthercomprising first and second feet arranged to extend underneathrespective posterior condyles and rest against posterior condylarsurfaces of the femur, and the second component comprises an extensioncomponent; a superstructure coupled to the extension component andarranged to slide parallel to the resected femoral surface towards andaway from the feet; and a stylus coupled to the superstructure andarranged such that when the first surface of the foot component restsagainst the resected femoral surface a tip of the stylus extends overthe femur such that sliding the superstructure towards the feet causesthe stylus tip to contact the anterior cortex of the femur; wherein thesuperstructure further comprises a first guide hole defining a firstalignment axis extending into the resected femoral surface at apredetermined distance from the level of the stylus tip in the plane ofthe resected femoral surface; and wherein the extension componentdefines a second guide hole defining a second alignment axis extendinginto the resected femoral surface such that the distance between thefirst and second guide holes varies as the superstructure slidesrelative to the body.